Joker in DK has this Chelsea grin thing going on no one knows why.
Here is my thought, Jack was a comedian a very poor one at that. He at one show made a joke about a local don. Said don was TICKED. He had him captured and given the grin for punishment. The pain drove him to madness he lost sight of reality and he thinks what he is doing is a game...for fun. He is crazy because of one joke...his killing joke if you will.